The present invention relates to sheet feeding apparatus in general, and more particularly to improvements in apparatus for removing discrete sheets from a pile or stack of registering sheets. Still more particularly, the invention relates to improvements in sheet feeding apparatus which can be utilized with advantage for singularizing and otherwise manipulating successive sheets of a pile or stack wherein the neighboring sheets exhibit a pronounced tendency to adhere to each other, especially as a result of electrostatic charges. Typical examples of such sheets are dielectric receptor sheets which are used for exposure to object-modulated X-rays in the gas-filled interelectrode gap of an ionography imaging chamber. The invention also relates to a radiographic system which embodies the improved sheet feeding apparatus.
In many presently known radiographic systems which employ ionography imaging chambers, the dielectric receptor sheets and sheet-like carriers therefor consist of polyester or the like. A radiographic system which employs such sheets is disclosed, for example, in commonly owned copending application Ser. No. 693,912 filed June 8, 1976 by Pfeifer et al., now U.S. Pat. No. 4,061,915 granted Dec. 6, 1977, to which reference may be had, if necessary. The arrangement which is disclosed in the application of Pfeifer et al. employs several trays for differently dimensioned dielectric receptor sheets and a further tray for sheet-like flexible carriers consisting of polypropylene. The receptor sheets as well as the carriers are elastically deformable but relatively stiff and the receptor sheets are highly sensitive, i.e., their surfaces are likely to be scratched or otherwise damaged in response to shifting of neighboring sheets relative to each other. In other words, removal of sheets from the respective stacks should be carried out with utmost care, preferably with a minimum of sliding movement of neighboring sheets with respect to each other. In fact, it is even desirable to avoid placing a substantial weight on any of the sheets in a stack. Furthermore, it is desirable or advantageous to avoid the use of suction heads, cups or like sheet attracting devices wherein the pressure of air is very low because strong adherence of a suction cup to a sheet is likely to damage its sensitive surface. This evidently conflicts with the need for reliable singularization of sheets, i.e., with the need to insure that sheets are removed individually rather than in groups of two or more. Therefore, presently known sheet feeding apparatus are not satisfactory or are not sufficiently satisfactory for reliable removal of individual dielectric receptor sheets or the like from a pile or stack.